This invention relates to a low temperature method of converting silica precursor coatings to ceramic silica coatings. The method comprises applying a silica precursor coating to a substrate, exposing the coating to an environment comprising ammonium hydroxide and/or wet ammonia vapors and subjecting the coating to a temperature sufficient to yield the ceramic coating. The methods of the invention are particularly applicable to applying coatings on electronic devices.
Researchers have recently shown that thin film ceramic coatings on electronic devices and circuits are valuable for their protective and dielectric effect. As protective agents, these thin films can assure reliability and extended service life of the electronics under a variety of environmental conditions and stresses such as moisture, heat and abrasion. As dielectric agents, these films can inhibit electrical conduction in many applications such as in multilayer devices where they function as interlevel dielectrics.
The art suggests several methods for the application of these ceramic coatings. For instance, Haluska et al. in U.S. Pat. Nos. 4,749,631 and 4,756,977, which are incorporated herein by reference, disclose silica based coatings produced by applying solutions of silicon alkoxides or hydrogen silsesquioxane, respectively, to a substrate and then heating to temperatures of 200.degree.-1000.degree. C. These same references also describe the application of other silicon-containing coatings such as silicon carbide, silicon nitride or silicon carbonitride onto the initial silica layer for added protection or dielectric effect. The ceramic coatings produced thereby have many desirable characteristics such as microhardness, moisture resistance, ion barrier, adhesion, ductility, tensile strength, low electrical conductance and thermal expansion coefficient matching that make these coatings especially valuable.
Haluska et al. in U.S. Pat. Nos. 4,847,162 and 4,842,888 also teach the formation of nitrided silica coatings by heating hydrogen silsesquioxane resin and silicate esters, respectively, to a temperature of between 200.degree. and 1000.degree. C. in the presence of ammonia. These references, however, teach that the ammonia should be anhydrous and that the resulting coating has about 1 to 2% by weight nitrogen incorporated therein.
Glasser et al. in the Journal of Non-Crystalline Solids, 64 (1984) pp. 209-221 teach the formation of ceramic coatings by heating tetraethoxysilane in the presence of ammonia. As with Haluska '162 above, however, this reference also teaches that the ammonia should be anhydrous and that the resultant silica coatings are nitrided.
Despite the efficacy of the above coatings, ceramification at temperatures less than 400.degree. C. is so slow that commercial applications are impractical. Utilizing temperatures in excess of 400.degree. C., on the other hand, can destroy various temperature sensitive devices. Therefore, a need exists for a method of rapidly applying ceramic coatings at low temperatures.
Jada in U.S. Pat. No. 4,636,440 discloses a method of reducing the drying time for a sol-gel coated substrate comprising exposing the substrate to aqueous quaternary ammonium hydroxide and/or alkanol amine compounds. The methods of this reference, however, are different than that disclosed herein in that Jada requires the coating to be dried prior to heating. Moreover, Jada is specifically limited to hydrolyzed or partially hydrolyzed silicon alkoxides and fails to teach the utility of the process on a coating of hydrogen silsesquioxane. As such, it is surprising and unexpected that the use of wet ammonia vapors and/or ammonium hydroxide, as taught herein catalyzes both the hydrolysis of Si--H bonds to Si--OH bonds and the condensation of Si--OH bonds to Si--O--Si.
The present inventors have now discovered that by exposing a silica precursor to ammonium hydroxide and/or wet ammonia vapors. a ceramic silica coating can be obtained on various substrates, including electronic devices, at temperatures as low as room temperature.